From time to time I receive emails asking whether there are any brands of fluorocarbon line at the local Cabela's or Bass Pro Shops that would work for tenkara lines.
My answer to that is "Yes, but..." You can buy a line you can use. It just won't be as good as the lines that are sold as tenkara lines
The main advantage of fluorocarbon over nylon mono is the density - which allows you to cast a smaller diameter (and lighter) line. Any fluorocarbon line is going to give you the greater density, but fluorocarbon lines are not all the same.
The lines sold to US bass anglers have been formulated to be limp, so that they stay on a spinning reel rather than coming off in wild coils - which the stiffer fluorocarbon lines definitely want to do. Fluorocarbon tippet material, although available in the right sizes - 0X, .013" or .015" is even limper. For tenkara, though, the stiffer lines cast better.
A while ago, I got an email from a guy who had specifically asked about the Stren Clear Blue Fluorescent fluorocarbon I mention on the Level Line page. He really wanted to buy a whole spool of line, and could get it locally at Bass Pro Shops. I told him it would work and recommended either 12# or 15# test for his 12' Iwana. He wrote back and said he got the 12# and that it worked well (he also outfished his friend 27 to 8 - he used Ishigaki kebari, chosen at random without regard to hackle color - his friend used powerbait). I just love reports like that!
The Stren fluorocarbon casts better than a level mono line, and I prefer level fluorocarbon to furled lines made from mono, thread or kevlar, but I do believe the orange or chartreuse Hi-Vis Fluorocarbon line casts better than the Stren. Even more important, though, the brightly colored fluorocarbon lines are so much easier to see. That is extremely important in being able to detect subtle strikes.
I guess where I come out on all this is to ask "how much line do you need?" If you want a whole spool, the Stren is as good a choice as you'll find. But if you don't need 200 yards the lines specifically sold as tenkara lines really do work better.
And yes, the guy who bought the Stren eventually bought some of the orange Hi-Vis fluorocarbon, and I really don't think he is going to go back to the Stren.